The present invention relates to the field of internal combustion engines with methane injection system.
The increasingly strict limitations posed by laws on the noxious emissions from internal combustion engines have originated new interest by manufacturers in engines with methane feed.
According to the conventional art, methane engines are provided with a choke device for metering the methane to be mixed with the air fed to the engine, according to an arrangement similar to that of a usual carburettor of a gasoline engine. However, this solution is no longer acceptable since it cannot be used in conjunction with an electronic system for controlling the operation of the engine which is able in particular to control the metering as a function of the composition of the exhaust gasses as detected by a so called lambda probe.
Therefore, it would be desirable to provide an engine fed by methane having an injection feed system.
A further problem lies in the need of adapting a gasoline injection engine or a diesel engine so that they can be fed by methane. It is to be noted that because of the gaseous state of the methane, the volumes involved are about ten times greater than those of a gasoline injection system or an injection system of a diesel engine. This means that an injection device for the injection of methane is of a size much greater than an injection device for gasoline or diesel fuel. As a consequence of this, the structure of a gasoline injection engine or a diesel engine cannot be immediately used for the injection of methane. In particular, the seats of the injection devices of a gasoline engine or a diesel engine are too small for receiving the methane injection devices. On the other hand, it would be desirable to transform a gasoline or diesel engine into a methane engine with no modification to the structure and geometry of the engine.
The object of the present invention is that of solving satisfactorily all the above mentioned problems while providing in particular a methane injection engine starting from a gasoline injection engine or a diesel engine with no substantial modification to the structure of the engine.
Another problem which is encountered in methane fed engines is the following.
In the above indicated engines, a basic operation parameter for ensuring regular operation in terms of homogeneous distribution of the air/methane mixture cylinder by cylinder is constituted by the timing in which injection begins.
In conventional engines with direct gasoline injection, the injection device associated with each cylinder is controlled for carrying out the gasoline injection when the intake valve of the associated cylinder is still closed. This is done to allow to the gasoline the time for mixing with air (flight time) before entering into the cylinder.
Studies and tests conducted by the applicant have shown that with internal combustion engines where methane is fed by injection cylinder by cylinder (multi-point injection) the above mentioned way of operation is an origin of drawbacks. Indeed, given that methane is injected into the intake conduit of each engine cylinder in a gaseous state, it occupies a volume greater at least by two hundreds times that of an equal mass of gasoline injected in a liquid state. This implies that if the methane is injected into a cylinder when the intake valve is closed, it can be sucked in the conduit of another cylinder that during this stage has its intake valve opened. The low inertia of a gas with respect to a liquid and the volume occupied within the conduit are the main reasons for which the above mentioned phenomenon takes place if a gas is injected and does not take place if a liquid is injected. This re-flow phenomenon implies a mis-distribution of the air/methane mixture cylinder by cylinder and therefore a fluctuation of the concentration of unburned gasses (in particular carbon monoxide and oxygen) in the exhaust gasses, which prevents a feedback control by a lambda probe or the composition of the exhaust gasses in order to improve the efficiency of the catalytic converter.
A further object of the invention is that of solving this drawback.